An intracavity frequency-doubled laser generally comprises a pump-light source, first and second mirrors forming a laser resonator, a gain-medium for providing a primary laser-beam at a fundamental wavelength, and an optically-nonlinear crystal. The first mirror, gain-medium, optically-nonlinear crystal and second mirror are arranged on a longitudinal axis of the laser resonator. The second mirror may serve as an outcoupling-mirror for the resonator. Interaction of the primary laser-beam with the optically-nonlinear crystal creates a second laser-beam (referred to hereinafter aa a frequency-multiplied beam) having two or more times the frequency the primary laser-beam. By way of example, such an interaction can create green light from infrared light.
In one conventional laser of this type, the primary laser-beam comprises between about five to twenty longitudinal modes. These longitudinal modes are very closely-spaced oscillation-frequencies of the laser. Inclusion of an optically-nonlinear crystal within the laser resonator leads to strong fluctuation of the amplitude of the resulting frequency-multiplied beam, for example, with up to one-hundred percent (100%) modulation depth.
This amplitude fluctuation, which can be simply described as noise, has four primary sources. A first of these sources is interaction between frequency-multiplied laser-light created by the primary laser-beam travelling in one direction with frequency-multiplied light created by the primary laser-beam travelling in the opposite direction, after reflection from the outcoupling mirror. This interaction leads to interference and standing waves, and is referred to hereinafter as the "first noise-source".
A second of these interaction or noise-sources arises from interaction between the primary laser-beam with a portion of the frequency-multiplied beam which, in passage through the optically-nonlinear crystal, is reconverted to laser light of the fundamental wavelength. This interaction can significantly influence operation of the laser at the primary oscillation-frequency (about which the mode oscillation-frequencies are closely grouped) leading to chaotic fluctuations. This noise-source, hereinafter defined as the "second noise-source", is discussed in a paper "Elimination of chaos in a multilongitudinal-mode, diode-pumped, 6W intracavity doubled Nd:YAG laser", M. Tsunekane et al., Opt Lett, Vol. 22, No. 13 (July 1997).
The outcoupling mirror is less than one-hundred percent transparent for, i.e., reflects some portion of, the frequency-multiplied beam. That portion of the frequency-multiplied beam which is reflected by the outcoupling mirror can interfere with the original frequency-multiplied beam, thereby contributing to noise in the laser. This is the "third noise-source".
In the optically-nonlinear crystal, different longitudinal modes come into interaction with each other through sum and difference frequency formation. This can effect chaotic power fluctuation. In a linearly-configured laser with standing waves (standing-wave oscillator) this effect is further supported by so-called "spatial holes", i.e., residual amplification in the active gain-medium which remains at the nodes of corresponding oscillation modes. This effect is discussed in a paper "Large amplitude fluctuations due to longitudinal mode coupling in diode pumped intracavity doubled Nd:YAG lasers", T. Baer, J. Opt. Soc. Am. B, Vol. 3, No. 9 1175-1180 (Sept 1986). The effect is described hereinafter as the "fourth noise-source", and can be reduced by a suitable mode distribution as disclosed in U.S. Pat. No. 5,627,849.
To avoid or minimize this fourth noise-source, a single-mode (single-frequency) ring-laser oscillator configuration can be selected, or a standing-wave oscillator with an intracavity, frequency-selective element, such as an etalon, can be selected, thereby excluding nonlinear coupling of longitudinal modes. Such a ring-laser configuration is described in U.S. Pat. No. 5,052,815. Alternatively, the noise can be distributed over very many longitudinal modes, for example, about one-hundred, so that the modulation depth of the noise is less distinct. For this purpose, a laser with a very long resonator must be used.
Formerly, the above-discussed four noise-sources have been only isolated, that is, in part, not even considered, and measures for minimizing the noise have been confined primarily to the fourth noise-source. Further, the known solutions for the problem have the disadvantage, as discussed above, that a particular laser-configuration is necessary, for example, a ring-laser; that supplementary optical elements are necessary; or that the resonator must be made very long, requiring costly temperature stabilization arrangements. Accordingly, there is a need for a solution to the problem of providing an intracavity frequency-multiplied laser with strongly-reduced noise which does not require that the laser be driven in a single mode, many longitudinal modes, or with any particular mode-distribution.